Support repository-local `mcp.json`

[strawgate]

Right now MCP servers are added at compile time -- it would be nice if there was an option to spin up the mcp servers provided in a repository's mcp.json or similar runtime mcp server option

[pelikhan]

We could do:

mcp-servers:
  foobar:
    source: ./mcp.json

The reason we don't blanket pull mcps is that a bunch of them are dev solo only.

[strawgate]

mcp.json would normally hold multiple mcp servers which might make putting it under a name key awkward -- perhaps a reserved from-repo key or something similar?

[pelikhan]

As a workaround you can store MCP server definitions in shared agent workflows right now. They will be merge into the set.

[strawgate]

Yeah, i'm doing this now https://raw.githubusercontent.com/elastic/ai-github-actions/refs/heads/main/.github/workflows/gh-aw-fragments/elastic-tools.md

I'm making a central repository of gh-aw actions that can be workflow_call'd -- the downside is that the only things I can make configurable for consumers are runtime configurations -- anything which is compile time must be static and I can't make configurable with inputs in the workflow_call.

The other side effect of workflow_call-ing is that the repository with the remote workflow is not cloned locally and so #16511 is key for it to work (which I just have implemented in my fork at the moment).